Update: NASA's Kepler space telescope has suffered a hardware malfunction threatening to end its life, a potentially sad finale to an important mission. This gallery of its greatest hits ran in November 2012, when the telescope completed its initial mission and prepared for a four-year extension. We are again offering this look at its best work, along with a few updated exoplanets it found in the meantime.

After three and a half years, NASA’s prolific planet-hunting Kepler space telescope is getting ready to enter its extended mission.

Over the last two decades, astronomers have discovered many extrasolar planets but each new system has been startlingly different than the last. Kepler was built to figure out the overarching properties of planetary systems by uncovering thousands of new planets. In particular, its goal is to figure out exactly how many stars in our galaxy have Earth-sized planets orbiting in a place where liquid water, and possibly life, could exist on the surface. Launched in 2009, Kepler released its first scientific findings a few months later.

Kepler hunts out planets by staring really hard at a star. Every once in a while, the instrument may catch a little blip where the star’s brightness decreases fractionally. It goes, 'Wait, did I just imagine that or was that a real thing?' If the telescope can see that tiny decrease in light on a regular basis, it might indicate a planet passing in front of that star and blocking out its light. After many follow-up observations, Kepler can confirm if a planet is really there and even determine some of its properties, such as its size.

Because it is looking to get good statistics on the percentage of stars with planets, Kepler is meticulously watching about 150,000 stars simultaneously. Using this method, the telescope has identified more than 2,300 potential candidates and confirmed the existence of more than 100 planets. Astronomers estimate that around 5 percent of all stars have Earth-sized planets.

Though scientists thought that by now they would have bagged their prize – an Earth-sized planet in its star’s habitable zone – the mission instead proved that stars are noisier than previously thought. The extended mission of up to four years will ensure that this remarkable instrument will zoom in on ever more planets and, very likely, one that resembles this blue marble we call home.

Here, we take a look at Kepler’s last three years of successful exoplanet hunting and some of the best discoveries it has made. We look forward to many more exciting findings and, of course, many more awesome "bullshit artist renderings."

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First Planets

Everybody’s got to start somewhere. For Kepler, it all began with five planets that were announced in 2010. The exoplanets were numbered and named Kepler 4b, 5b, 6b, 7b, and 8b, all discovered around different stars. All are known as “hot Jupiters” since they have masses comparable to the largest planet in our solar system and orbit very close in to their parent stars, with years shorter than five days. Though these worlds are unlikely places to find Earth-like life, they represented the Kepler mission team’s humble beginnings.

Two Planets, One Star

Continuing Kepler’s series of early firsts were the two planets found orbiting the star Kepler-9. Kepler 9b and 9c were in the mission’s first multi-planet system. The Saturn-size planets orbited their parent star in about 19 and 38 days, respectively. A third, super-Earth-sized planet was later confirmed to be in the system as well.

Kepler has since discovered many stellar systems that approach the eight planets of our own, with the record holder being Kepler-11 with its six strange planets.

Rocky World

Kepler is always on the lookout for the most Earth-like planets it can spot. Kepler-10b was the first world confirmed to be rocky like our own. The planet has a radius about 1.4 times that of Earth and orbits its parent star in less than a day. Kepler’s super-accurate measurements were able to determine that the world has a mass about 4.6 times that of Earth, making it similar in density to an iron dumbbell. Given its composition and close proximity to its star, some scientists consider Kepler-10b to be more of a super-Mercury than an Earth-twin.

First Earth-Size Planets

The main goal of Kepler is to figure out just how many stars have Earth-size planets in their habitable zone. Which is why the mission passed a major milestone in December 2011, when scientists announced that Kepler had spotted its first Earth-sized planets, Kepler-20e and Kepler-20f. These not-quite-doppelgangers had radii roughly 0.87 and 1.03 times the Earth’s but both orbited far to close to their host star to probably be habitable. They both can be found in the same five-planet system about 1,000 light-years away in the constellation Lyra.

Possible Ocean World

So far, Kepler-22b is the closest things we’ve got to home. The planet, which is 600 light-years from Earth and has a radius about 2.4 times that of our own planet, orbits a sun-like star. Furthermore, its year is about 290 days, slightly shorter than our own year. Scientists don’t know its composition, but if it has an atmosphere, it may have a giant balmy ocean and potentially, life.

The Evaporating Planet

In May, Kepler spotted the enormous comet-like tail of a smoldering planet around a star 1,500 light-years away. The star is smaller and cooler than our sun and the small planet is Mercury-size. Orbiting every 16 hours around the star, the planet is slowly disintegrating into dust. The star-facing side could be an ocean of seething magma. At the current rate of evaporation, the planet will be completely destroyed in about 200 million years.

Tiniest Planets

Kepler-42 contains the cutest little solar system ever seen. The red dwarf star is host to three rocky planets that are all smaller than the Earth, with the smallest being roughly Mars-size. All the planets orbit the star in less than two days, meaning that they are all likely far too hot to hold any life.

Tatooine World

Scientists looking to convey their discoveries in layman’s terms couldn’t do much better than Kepler-16b. The planet was the first to be discovered orbiting two stars, much like Luke Skywalker’s home world Tatooine in the Star Wars movies. Located about 600 light-years from Earth, the planet was not one that astronomers were sure could exist. Gravitational interactions between binary stars could just as likely destroy planets as make them form. The Saturn-size world orbits its two parents every 229 days and while it may not host living things, any potential moons might.

Kepler has since shown that planets around binary stars are not cosmic oddballs. Kepler-34b and 35b are other planets orbiting two stars and the binary pair Kepler-47 even hosts a multi-planet system.

Quadruple Star System

If binary stars with planets aren’t your thing, why not try quadruple-star-systems with planets? In October, NASA announced the discovery of PH1, a planet found using Kepler data -- but not by astronomers. Instead, a citizen science group called Planet Hunters used thousands of eyes from volunteers to scan Kepler’s transit lines. These productive amateurs determined that PH1 was a Neptune-size planet that orbits its two host stars every 137 days. The double parent stars were in turn orbited by a second binary stellar system.

Smallest Exoplanet

Shattering all records, Kepler found an exoplanet just barely bigger than the Earth's moon in February 2013. Kepler-37b was half as small as the previous title-holders and may be one of the tiniest worlds the telescope will ever detect.

Image: NASA/Ames/JPL-Caltech

Habitable Super-Earths

One of Kepler's last major discoveries before its hardware malfunction was also one of its greatest. Not one, not two, but three exoplanets in the habitable zone, the area around a star in which liquid water and perhaps life could exist. Kepler-62e, Kepler-62f, and Kepler-69c are all worlds somewhat bigger than Earth. Because we have no examples of worlds such as this in our solar system, it is hard to say exactly what the conditions on them are. But all could theoretically have balmy weather and might one day be a great vacation spot.

Image: NASA/Ames/JPL-Caltech

Kepler’s Stars

Though a planet-hunting mission, Kepler has been a boon to astronomers that study stars. We only have one real close-up example of a star -- our sun – but it can’t tell us about all the variability and extreme cases that other stars may possess. But because Kepler has to know the characteristics of the stars it studies with incredible detail, it has provided incomparable data to heliophysicists. Kepler has been able to witness the delicate star-quakes that shiver through distant stellar atmospheres, which provide information about fundamental properties, such as size, age, mass, and internal structure. Though the mission may be remembered for the planets it discovers, its contribution to our understanding of the universe is just as important.

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